The present invention is an improvement upon commonly used back EMF detection methods for determining the proper commutation of currents in a brushless DC motor. In general, these well known methods detect the back EMF by monitoring the motor phases. The points in time at which the back EMF signal crosses from a positive amplitude to a negative amplitude, known in the art as a zero-crossing, are determined and from this information the commutation order and speed is determined.
The phase shift or lead angle between the back EMF and phase-current signals of corresponding phases is an important factor to the quality of the commutation control. It has, for example, an impact on the instantaneous motor torque and the vibrations developed by the motor. The optimum value of this lead angle depends on the running mode of the motor; specifically, start, acceleration, steady state, braking. Consequently, the lead angle must be variable.
The back electromotive force (back EMF) is a movement induced voltage (French expression: tension induite de mouvement). It is therefore determinative of the geometrical position of the rotor. Thus, detecting the back EMF zero crossings for each phase enables the commutation control circuits to know the speed and position of the rotor and tailor the commutation rate, or more specifically, the lead angle dependent instant of commutation, to the current state of the motor. It is very important that the back EMF signal be detected without error.
However, it is well known in the art that commutating the motor phases will cause electrical noise. The noise is usually in the form of phase tap voltage spikes which can be large enough to cause anomalous zero-crossing detection by the back EMF monitoring apparatus.
Heretofore, attempts have been made to mitigate the effect of the spikes by filtering the phase tap voltage signal. However, filtering also distorts the zero-crossing timing and may lead to complicated circuitry.
It is very important that the commutation control apparatus commutates the motor phases prior to the alignment of the rotor with the energized phases. In other words, the commutation control apparatus maintains the lead angle between the rotational position of the magnetic wave and the physical rotational position of the rotor. The magnitude of the lead angle is important in controlling motor acceleration. During braking, a lagging angle is maintained. In some applications, the magnetic wave and physical rotational position of the rotor are synchronous.
The objective of the present invention is to provide a simple means to eliminate the effects of voltage spikes caused by phase commutation upon back EMF detection apparatus.
Another object of the present invention is to provide a means by which the lead angle of the commutation is controlled.